Excretion of trans,trans-muconic acid (2,4-hexadienedioic acid; t,t-MA), a potential biomarker of low-level exposure to benzene, was determined in 32 smokers and 82 nonsmokers. In smokers the median background excretion of t,t-MA was 0.13 (0.06-0.39) mg/g creatinine and was significantly higher (P < 0.05) than the value of 0.065 (0.02-0.59) mg g creatinine in nonsmokers. For nonsmokers, the correlation between t,t-MA excretion and environmental exposure to benzene in ambient air, which was determined during the 8-day study period by personal diffusion samplers, was not significant (r = 0.164, P = 0.18). Nonsmokers living in the city tended to have higher t,t-MA excretion rates than nonsmokers living in the suburbs. However, the difference was only significant for nonsmokers from nonsmoking homes. For the same location (suburb or city), smoking at home leads to a marginal increase in t,t-MA excretion of the nonsmoking members of the household. In a further study with eight nonsmokers we found that dietary supplementation with 500 mg sorbic acid significantly increased (P < 0.001) the mean urinary t,t-MA excretion from 0.08 (0.04-0.12) to 0.88 (0.57-1.48) mg/24 h. Under study conditions 0.12% of the sorbic acid dose was excreted in urine as t,t-MA, thereby indicating that a typical dietary intake of 6-30 mg/day sorbic acid accounts for 10-50% of the background t,t-MA excretion in nonsmokers, and for 5-25% in smokers. As a consequence, sorbic acid in the diet is a significant confounding factor in assessing low-level benzene exposure if t,t-MA excretion in urine is used as a biomarker.